1. Field of the Invention
The present invention relates, in general, to foamed, molded, uncrosslinked thermoplastic articles. More specifically, a method and apparatus for producing foamed, molded, thermoplastic shoe midsoles are disclosed.
2. Description of the Prior Art
In the production of foamed articles of thermoplastic resin, three methods are generally known (1) the bead molding method; (2) the injection molding method; and (3) the extrusion method. Under the bead molding method, pre-foamed particles are placed in a mold cavity and heated to encourage further expansion and fuse-bonding of the particles. The bead molding method is disadvantageous because it requires two or more processing steps and traces or marks of the beads are left in the resulting molded article.
Under the injection molding method, a molten mass of a foamable resin is injected into a mold cavity and the injected resin is allowed to expand in the mold. This method requires a high injection pressure, however, and hence, a large and strong molding apparatus capable of withstanding the pressure. Moreover, the injection molding can only provide an expansion ratio (i.e., density reduction from resin to foam) of at most about 2.
In the extrusion method, a molten, expandable resin is extruded through a die. The method may only be used to produce foamed articles of a simple shape such as sheet- or rod-like products, however.
One application where the above-noted problems of the foaming/molding prior art are especially visible is in the production of athletic shoe midsoles (i.e., the material between the shoe upper and the ground-contacting outer sole). Shoe midsoles have been characterized as the most important portion of athletic footwear. The rather significant forces generated by runners as they run, especially in the ball, forefoot and heel regions of the foot, must be largely absorbed by the shoe midsole. Furthermore, the midsole preferentially is also capable of returning a significant portion of the runner's energy through his/her body as the shoe contacts the ground, creating a beneficial sensation of springiness. Athletic shoe midsoles must also be able to withstand the large number of compression and return cycles generated by, for example, long-distance runners, without jeopardizing the weight bearing and cushioning capacity of the shoe midsole.
Specifically, material utilized for an athletic shoe midsole must exhibit the requisite levels of hardness, resiliency and compressive strength. Hardness is commonly measured by, for example, an ASKER C hardness tester (or durometer). The hardness tester calculates the hardness of a test specimen from the measured depth of penetration of an indentor of predetermined geometry into the specimen (once a state of balance is reached between the resistance force of the specimen and the force applied to the indentor). To be suitable for use as an athletic shoe midsole, thermoplastic foamed material must exhibit a hardness of 30 to 70 ASKER C, and preferably exhibits a hardness of about 40 to 55 ASKER C.
The resiliency of a material may be quantified by measuring the material's energy return ratio. In general, the energy return ratio is obtained by dropping an object onto the material and measuring how high the object bounces back (e.g., a perfect spring would have an energy return ratio of 1.00). The methodology of measuring a material's energy return ratio is discussed in detail in U.S. Pat. No. 4,984,376, which is hereby incorporated by reference, at column 10, lines 37 to 64. To be suitable for use as an athletic shoe midsole, a material should exhibit an energy return ratio of at least 0.20 (using the method of measuring energy return ratio disclosed in the ASTM bulletin number D-2632-79). By way of comparison, under this testing procedure, foamed thermosetting polyurethane exhibits a energy return ratio of about 0.25 to 0.30 and foamed HYTREL.RTM. (a polyester elastomer manufactured by E. I. du Pont de Nemours and Co. of Wilmington, Del.) exhibits an energy return ratio of about 0.50 or more.
Compressive strength is measured by gradually compressing a flat sample of material (e.g., a cube with a 10 cm.times.10 cm (or 1 inch by 1 inch) top surface) and measuring the pressure needed to compress the sample a given proportion of its original height (e.g., 10%, 25% and 50%). Compressive strength is measured in kilo Pascals (kPa), or pounds per square inch (psi). Preferred materials for athletic shoe midsoles should exhibit a compressive strength of about 48 to 138 kPa (7 to 20 psi) at 10% compression, 117 to 207 kPa (17 to 30 psi) at 25% compression and 248 to 379 kPa (36 to 55 psi) at 50% compression.
Another important criteria which any proposed athletic shoe midsole material must meet is the material's specific gravity. Specific gravity relates to, and in some senses, grows out of the previously discussed properties. To be suitable for use as an athletic shoe midsole, a material must have a specific gravity of about 0.5 gm/cm.sup.3 or less. Preferred midsole materials have a specific gravity of about 0.3 gm/cm.sup.3 or less. This restriction, in turn, limits the methods which may be used to form the midsole. For example, injection molding may typically only be used with materials having higher specific gravities than those suitable for use as athletic shoe midsoles (e.g., about 0.8 gm/c.sup.3). If lower density materials are injection molded, the material will often not foam uniformly, thereby causing broken cells within the foamed product.
In addressing these concerns, the athletic footwear industry has developed a variety of different solutions. For example, many shoe midsoles are currently made of crosslinked EVA (ethylene vinyl acetate). Crosslinked EVA exhibits good durability, but since it is a crosslinked material, EVA generates a large amount of non-recyclable waste material during processing. Furthermore, production of midsoles from crosslinked EVA normally requires several processing steps (see, e.g., U.S. Pat. No. 4,900,490, which is hereby incorporated by reference). For example, after a plank of EVA is produced, the plank must be skived (i.e., cut along its height to form two or more separate, thinner planks). Thereafter, the plank is cut into plugs having the approximate configuration of the desired midsole. The plugs of EVA are then inserted into molds and compression molded. The plugs are purposely cut slightly oversize relative to the molds to encourage the material to adapt to any configurations present within the mold.
The compression molding step also re-forms a skin over the open cells of material which were exposed when the plank of EVA was skived. This processing methodology is both multi-step and time-consuming (e.g., 5 to 10 minutes per compression cycle--i.e., heating to seal the skived EVA and allowing the re-compressed EVA to cool).
Other currently-available processes also exhibit several problems. For example, in producing shoe midsoles from thermoplastic material (e.g., polyester elastomer), multi-step processing is still the norm. For example, a large piece of thermoplastic material is extruded. Thereafter, the material is skived, die cut into plugs of approximately the desired size and the plugs of material are subjected to secondary compression molding to form designs in the material and to create a cell-enclosing skin over the cut areas of foam.
When uncrosslinked thermoplastic materials are utilized, the large amounts of waste material generated by this type of process may at least be recycled (with crosslinked material, the excess material cannot be reprocessed and must be discarded), but even with uncrosslinked materials, these multiple processing steps still mandate that large amounts of labor be expended in producing each foamed article. Furthermore, since the thermoplastic material is normally fully foamed when it is subjected to the secondary, skin-forming compression molding, it is difficult to produce articles having intricate areas and/or shapes of raised material (e.g., company logos on the sides of athletic shoe midsoles). Conventional foaming methods also have difficulty producing foamed material having substantially uniform density and cell structure throughout the article (e.g., in the expansion process of U.S. Pat. No. 4,806,293, which is hereby incorporated by reference, the expanding material may fold over on itself, thereby forming YMP seams in the finished article).
The present method and apparatus, on the other hand, favorably resolve these problems and suboptimizations inherent in the prior art by providing a one-step process for producing low-density foamed articles from uncrosslinked (and hence recyclable) thermoplastic material whereby uniform density is maintained, foam cell integrity is maintained and even intricate designs in the female mold section may be reproduced in the shoe innersole.